1. Field of the Invention
The present invention relates generally to an improved polysilicon resistor and method for making the same.
2. Description of the Related Art
Various types of polysilicon resistors have been known. However in making polysilicon resistors in the past several problems and deficiencies have been noted. Specifically a trimming of the resistance value either by lasers or by having multiple resistors and having to "blow" links have been needed to obtain high accuracy in the resistance value. Further the resistors that are formed have first and second order temperature coefficients (TC1 and TC2) in normal operation ranges that make them less than ideal.
Other attempts at other non-laser trimmed resistor such as Isobe, et al. U.S. Pat. No. 5,187,559 and Amemiya et al., U.S. Pat. No. 4,210,996 have been tried. However these also suffer from various problems as to the temperature coefficients and the methods of making them are difficult. As an example although the Amemiya et al. device allows for electrical trimming however the temperature characteristics do not lend themselves to ready usage. Further a device built along the lines Amemiya et al. can only be built using doping concentrations at higher than 1.times.10.sup.20 atoms/cm.sup.3. Consequently this device is less than desirable.
A device according to Isobe, et al. actually requires that two dopings occur for each resistor one with a positive TC1 and a second with a negative TC1 be used so that a zero TC1 resistor can be formed. This increases the complexity of formation of the device. Further both dopings are at high levels of concentrations, which create problems in the manufacturing process .